To increase light output from a lamp, a filament may be made larger, or longer. Larger diameter filaments are awkward to position, so in general filaments are made longer. The longer filament is then held in several places along its length for stable positioning. A common method holding the lengthened filament is to fold the filament back and forth and hold the end of each fold with a support wire. The opposite ends of the support wires are then coupled to an insulator, usually made of quartz or glass, called a bridge. The bridge extends or bridges between two bridge supports, usually metal rods. The metal rods may or may not provide the electrical connections for the two ends of the filament.
Currently bridges are made from solid, cylindrical quartz rods. The solid quartz rod is flame heated to a softened condition and then pressed onto the support rods and support wires. FIG. 1 shows a prior art solid rod bridge in cross section being heated by a flame. FIG. 2 shows a prior art solid rod bridge in cross section after being heated. Shading indicates the heat distribution. The bridge then molds around the support rods and wires, and after cooling should remain permanently positioned against them. The molding process results in a number of problems. Heating the entire mass of the bridge to pliability in the location where the molding takes place cannot be done quickly, and uniformly. As shown in FIG. 2, the heated side of the solid rod tends to be hotter and more pliable, while the opposite side tends to be colder and less pliable during the pressing. Only a fraction of a solid quartz rod is in a fully plastic state when the coil support wires and side rods are pressed. A fair portion, indicated by the shaded area in FIG. 2, of the rod is cooler and less pliable. Only, a limited portion of the bridge can then be spread up, down and around the support rod when the two are pressed together. A weak joint is then sometimes formed.
For an insufficiently melted bridge, the unmelted portion may also crack when pressed against the support rod. Even for a properly heated bridge, a thermal gradient exists across the bridge diameter, and residual stresses may be left in the bridge. The residual stresses may result in cracks on subsequent mechanical or thermal stress. When the bridge cracks, the lamp frequently fails. Another problem is that a sufficient length of the bridge needs to wrap around the support rod when melted to a pliable state. If the bridge is too short, or insufficiently melted, the melted bridge fails to wrap around the support rod and permanently couple with the support rod. High rework rates and scrap factors are the result of cracked or broken bridges. Lamp costs then rise. There is then a need for a better bond between the bridge and bridge support in incandescent lamps.